As an access scheme used in a wireless communication system such as a wireless LAN (IEEE 802.11) or Zigbee, there is a carrier sense multiple access with collision avoidance (CSMA/CA). According to this scheme, each terminal performs carrier sense of a channel before start of transmission, and transmits a packet after detecting that a channel is in an available (idle) state for a constant period. Here, the waiting constant period is a time obtained by adding a backoff time, which is based on a random value selected at random from a contention window (CW) range, to a given fixed time. In general, when the terminals each select the backoff time at random, collision can be avoided although the plurality of terminals simultaneously transmit packets. However, when the plurality of terminals incidentally select the same random value, a collision occurs. Therefore, in this case, each terminal reduces a collision probability by expanding the CW range, and then retransmits the packet. The CW starts from the minimum value CWmin and expands to twice every retransmission until reaching the maximum value CWmax.
Thus, in the CSMA/CA scheme, the CW increases with an increase in the number of retransmissions due to the collision or the like. Therefore, since the backoff time increases every retransmission, there is a concern that time delay caused due to the retransmission may be prolonged. For this reason, in an environment in which there are a considerable number of terminals and a collision occurrence frequency thus increases, it can be considered that a delay time up to transmission completion increases, and thus a request delay time may not be satisfied. As a method of resolving this problem, for example, a method has been suggested in which the problem of the unnecessary increase in the delay time caused due to overhead involving the backoff time is prevented by controlling the CW range according to the number of terminals or a traffic amount. Further, in the wireless LAN system, an EDCA scheme has been suggested in the IEEE 802.11e standard. In the EDCA scheme, the kinds of data are classified into four categories and the categories are each prioritized. For example, with regard to voice communication which is a category with short request delay and high priority, a fixed time (AIFS: Arbitration Inter Frame Space), CWmin, and CWmax are set to be short. Thereby, data belonging to a category with high priority is configured to be preferentially transmitted. According to the EDCA scheme, it is possible to prevent the time-out of data required to be less delayed by the priority control.
In the above-mentioned method, however, the control is merely performed so as to be suitable for a traffic amount relatively in consideration of the delay time, and the delay time between terminals in the environment of the same traffic amount is not guaranteed. Even in the EDCA scheme, a relative transmission priority order is merely controlled between data with different priorities. The delay time between the data with the same priority is not guaranteed.
Thus, in the conventional method, relative control is performed according to a traffic amount or the kinds of data. The problem remains in that the CW increases with an increase in the number of retransmission caused due to a collision or the like, and thus the backoff time increases every retransmission. Accordingly, the fundamental problem still remains in that the time delay caused due to the retransmission is prolonged. That is, since an operation is merely performed according to the normal CSMA/CA scheme in the same traffic amount or the same kind of data, delay guaranteeing control to prevent the time-out of the request delay of data has not been performed.